1. Field
The present application relates generally to the operation and design of analog front ends, and more particularly, to the operation and design of mixers for use in analog front ends.
2. Background
Transmitters typically use mixers to up-convert baseband signals to radio frequencies (RF). Receivers typically use mixers to down-convert RF signals to baseband signals. One type of mixer is a passive mixer that utilizes MOS switching devices. In a single transistor NMOS passive mixer, the signal dependent channel on-resistance is a result of the amplitude of both the input and output voltages, which results in signal dependent gain. The same can be said for a single transistor PMOS passive mixer. In a two transistor mixer having both PMOS and NMOS devices, the simultaneous enablement of both devices results in less overall signal dependent channel on-resistance due to the parallel combination of the PMOS and NMOS switch resistances. However, less than optimum mixer operation can occur when the local oscillator (LO) amplitudes are not equal and opposite or the process dependent on-resistances differ between the NMOS and PMOS devices.
The limitation of the operational voltage for MOS passive mixers results in a device specific limit for what may be considered acceptable linear operation. As the device geometries shrink so does the benefit that this architecture can provide. Basically there are two dominant constraints to limiting linear performance. The first constraint is that the voltage dependent resistance (channel of the MOS device) sets the lower limit of the MOS on-resistance. The second constraint is the parasitic activation of the MOS switch device (i.e., the device turns on when the signal is large thus clamping the signal's voltage for the region in time where activation occurs).
Therefore, it would be desirable to have a way to equalize the channel on-resistance of a MOS mixer to operate within the constraints described above.